IK1, also known as KCa3.1 or SK4, is a member of a family of small- to intermediate-conductance potassium channels in humans (SK1-3, IK1). IK1 plays a crucial role in T-cell activation by effluxing potassium to maintain a negative membrane potential that allows for additional calcium influx. IK1 is a potential therapeutic target (for inhibition) for autoimmune diseases and for organ transplant rejection. IK1 and SK1-3 are tetrameric and are gated/activated through calcium binding to calmodulin (CaM), which binds to a region within the cytoplasmic portion of the channels (CaM binding domain, CBD). A novel feature of IK1 activation is the additional requirement of phosphorylation of a specific histidine residue (His358) within the CBD by NDPK-B (nucleoside diphosphate kinase-B). Although histidine phosphorylation is well characterized in two-component signaling systems in prokaryotes, the extent and function of histidine phosphorylation in eukaryotes are just beginning to be understood. Recently, we hypothesized that histidine phosphorylation of IK1 is necessary to relieve metal- ion inhibition of the channel. In this application, biochemical, structural, and functional studies are proposed to elucidate the molecular mechanisms by which IK1 is regulated by calcium and histidine phosphorylation.